The first solar cells were fabricated in the mid 1950s from crystalline silicon wafers. At that time, the most efficient devices converted 6% of solar power to electricity. Advancements in solar cell technology over the past 50 years have resulted in the most efficient Si cell at 25% and commercial Si modules, an array of cells, at 10%. Although Si, in crystalline and polycrystalline forms, is the most common type of material used in solar cells, other semiconductors such as gallium arsenide, indium phosphide and cadmium telluride are being investigated for the next generation of higher efficiency solar cells. In particular, high efficiency structures such as tandem cells, in which multiple band gaps are layered in a single device, using GaInP, GaAs and Ge have attained record efficiencies of 34%.
Despite these impressive efficiencies, the high cost of manufacturing solar cells of the prior art limits their widespread use as a source of power. The construction of prior art commercial silicon solar cells involves four main processes: the growth of the semiconductor material, separation into wafers, formation of the device and its junctions, and encapsulation. For the cell fabrication alone, thirteen steps are required to make the solar cell and of these thirteen steps, five require high temperatures (300° C.-1000° C.), high vacuum or both. In addition, the growth of the semiconductor from a melt is at temperatures above 1400° C. under an inert argon atmosphere. To obtain high efficiency devices (>10%), structures involving concentrator systems to focus sunlight on to the device, multiple semiconductors and quantum wells to absorb more light, or higher performance semiconductors such as GaAs and InP, are needed. The gain in performance results in increased manufacturing costs, which stem from the multiplication of the number of fabrication steps. To date, these high performance architectures have been employed mainly for extra-terrestrial applications such as in space shuttles and satellites, where efficiency per unit weight is as important as fabrication costs.
Another problem with the solar devices of the prior art is the high cost of manufacturing materials. The amount of silicon needed for 1 kW of module output power is approximately 20 kg. At $20/kg, the material costs for electronic grade silicon is partially subsidized by the chip manufacturing sector. Other materials such as GaAs, which are synthesized with highly toxic gases, are a factor of 20 higher in cost at $400/kg. Because solar cells are large area devices, such material costs hinder the production of inexpensive cells. As a result, thin film devices, which have active layers several microns thick of amorphous Si, CdTe, and CuInSe2 are being explored. In 1991, O'Regan et al. reported the invention of a novel photochemical solar cell comprised of inexpensive TiO2 nanocrystals and organic dye, O'Regan et al. Nature 353, 737 (1991).
Bilayer devices, from spin casting a derivative of polythiophene on which a layer of C60 is evaporated, have been able to reach a maximum external quantum efficiency (EQE) of 23%. Higher efficiencies at 50% were obtained from blending derivatives of C60 and MEH-PPV into a homogeneous film for a single-layer device. Further improvements in efficiencies are limited by the poor electron transport properties of C60, which is characterized by hopping, and the low overlap between the device absorption and the solar emission spectrum, Greenham. N. C. et al., Phys Rev. B, Vol. 54, No. 24, December 1996.
It has been suggested previously to use CdSe particles in poly(3-hexylthiophene), see Alivisatos et al. Adv. Mater. 1999, 11, No. 11. This work only teaches the use of nanocrystals less than 13 nm in size and the devices produced do not approach the efficiencies of those of the instant invention. Further, this prior art admits solution chemistry problems with nanorods and offers no solutions to the problems solved by the invention described herein. Solar cells based on inorganic nanorods according to the instant invention, which have good transport properties and absorption spectra that can also be extended into the near infrared, can potentially reach efficiencies that rival conventional solar cells based on bulk inorganic semiconductors. It is the thin films incorporating semiconductor-nanocrystals according to the embodiments of this invention that provide solutions to the above stated problems.